A patent reference JP2005-312151A, hereinafter referred to as Patent reference 1, discloses a known stator for a rotating electrical device provided with a flange portion for attaching a motor housing thereat formed at an end portion in an axial direction of a retaining ring where cores are attached. The stator disclosed in Patent reference 1 is adapted for motors mainly for driving wheels of a hybrid vehicle. The Patent reference 1 discloses the stator for a rotating electrical device where the cores arranged in an annular form are retained by the retaining ring first then the retaining ring is attached to an inside of the motor housing.
In the rotating electrical device disclosed in Patent reference 1, the flange portion extends outward in a radial direction from a peripheral end of the retaining ring and a multiple number of through-holes are arranged circumferentially with a predetermined separation distance defined between the through-holes provided on the flange portion. After bringing the flange portion in abutting contact with end surfaces of boss portions of the housing, bolts inserted to the corresponding through-holes are retained to the boss portions by tightening the bolts thereat so that the stator is retained to the housing.
Ordinarily, the housing of the rotational electrical device adapted for installation to a hybrid vehicle or similar type of vehicles is formed of a lightweight metal, for example, an aluminum alloy, for reducing weight. On the other hand, the retaining ring where the cores are attached by force fitting or similar methods is formed with a material similar to that for forming the cores, the material having a small thermal expansion coefficient, in order to avoid lowering of the retaining force for retaining the cores due to a temperature change. As a result, the retaining ring, in most cases, is formed with a material similar to iron, or in other words, with a metal different from which the housing is formed of.
The temperature change in the rotating electrical device causes the retaining ring and the housing to move relative to each other. For example, when the temperature of the rotating electrical device rises, the housing formed of a metal with a large thermal expansion coefficient expands outward in the radial direction more than the retaining ring. As a result, the boss portions of the housing move relative to the through-holes extending through the flange portion of the retaining ring, which in turn may cause loosening of the bolts tightened to the boss portions due to loads in shearing directions acting on the bolts in the stator. Especially after the generation of a fretting wear on a surface of the housing due to the relative movement between the flange portion and the boss portions, tightening forces of the bolts rapidly become weak.
As a countermeasure for preventing loosening of the bolts, the bolts having a larger pitch diameter of thread may be used for increasing axial forces acting on the bolts. Nevertheless, when the pitch diameter of thread for the bolts is increased, the flange portion of the retaining ring and the size of the housing become larger, which in turn results in increase of the size of the rotating electrical device as a whole.
A need thus exists for a stator for a rotating electrical device and a stator retaining ring which are not susceptible to the drawback mentioned above.